Ceramic materials having negative coefficients of thermal expansion or which exhibit negative thermal expansion, i.e., which contract, rather than expand as their temperature increases, are known. In general, these materials have crystal structures with anisotropic thermal expansion, where expansion along one crystallographic direction is negative and expansion along a different direction is positive. Examples of ceramic materials exhibiting negative thermal expansion include lithium aluminosilicate (LiAlSiO.sub.4), cordierite (Mg.sub.2 A.sub.4 Si.sub.5 O.sub.18) and Ge-modified cordierite (Mg.sub.2 Al.sub.4 Si.sub.4.2 Ge.sub.0.8 O.sub.18) compositions, aluminum titanate (Al.sub.2 Ti.sub.2 O.sub.5) based compositions, calcium strontium zirconium phosphate (Ca.sub.1-X Sr.sub.X Zr.sub.4 P.sub.6 O.sub.24) and similar compositions, Zr.sub.2 P.sub.2 O.sub.9, ZrV.sub.2 O.sub.7, Ta.sub.2 WO.sub.8, and Nb.sub.2 O.sub.5. In polycrystalline form, these ceramics exhibit a "net" negative thermal expansion because the contractive component of thermal expansion is larger than the expansive component. A problem with known negative thermal expansion ceramic materials is that in many cases, the expansion anisotropy leads to microcracking, which reduces mechanical strength of sintered ceramics. There has been a need to develop suitable compositions and ceramic processing methods to allow the fabrication of high density and mechanically robust ceramic components of materials exhibiting negative thermal expansion behavior.
Of the various types of known negative thermal expansion ceramic materials, several ceramic compositions in the lithium aluminosilicate family exhibit the largest negative thermal expansion values. There are three identified negative-expansion compounds in the lithium aluminosilicate ceramic system: .beta.-eucryptite (LiAlSiO.sub.4), spodumene (LiAlSi.sub.2 O.sub.6), and petalite (LiAlSi.sub.4 O.sub.10). All of these compounds exhibit anisotropy in thermal expansion, with negative expansion in one crystallographic direction. Sintered .beta.-eucryptite ceramics exhibit the most negative thermal expansion, with reported thermal expansion values ranging from -6 to -8 ppm/.degree. C. (also known as 10.sup.-6 in/in/.degree. C.). The .beta.-eucryptite form of LiAlSiO.sub.4 is stable above 970.degree. C., whereas the .alpha.-eucryptite form is stable at lower temperatures. However, sintered LiAlSiO.sub.4 ceramics which are processed above 970.degree. C. always have the .beta.-eucryptite structure, because the transformation from the .alpha.-form to the .beta.-form that occurs during calcination (or sintering) is irreversible. Thermal expansion anisotropy of .beta.-eucryptite, with a hexagonal crystal structure, is very large, with .alpha..sub.a .about.+8 ppm/.degree. C. and .alpha..sub.c .about.-17 ppm/.degree. C.
.beta.-eucryptite ceramics can be readily formed using conventional ceramic methods of ball milling, calcination, and sintering. The LiAlSiO.sub.4 composition can be modified by additions of up to about 50 mol % of both AlPO.sub.4 and LiAlGeO.sub.4, without affecting .beta.-eucryptite phase formation or expansion behavior. With oxide starting materials, calcination temperatures of between about 1000 and about 1100.degree. C. and sintering temperatures of between about 1200 and about 1300.degree. C. have been used for the successful preparation of single-phase .beta.-eucryptite ceramics. There also has been some work in the synthesis of .beta.-eucryptite using sol-gel methods for powder preparation. The sol-gel method involves the use of expensive metal-organic precursors, but has some advantages related to controlling Li.sub.2 O volatility and achieving high sintered densities. However, anisotropic thermal expansion has made it difficult to produce high strength LAS ceramics. A relatively low modulus of rupture or flexural strength value of 2000 psi (13.8 MPa) is typical for .beta.-eucryptite ceramics.